Anderson Localization in Low-Dimensional Optical Lattices

TL;DR

This thesis provides a theoretical framework for understanding Anderson localization in low-dimensional ultracold atomic gases within optical lattices, exploring extended states, transport windows, and novel disorder simulation methods.

Contribution

It introduces analytical techniques for identifying extended states in correlated disorder and proposes experimental methods to generate and tune such correlations in optical lattices.

Findings

Extended states exist in finite systems within certain energy windows.

Correlated disorder can significantly reduce localization length.

Proposed methods enable tunable disorder and simulation of magnetic fields in optical lattices.

Abstract

Topic of the thesis is a theoretical description of the ultracold atomic gases in one- and two-dimensional optical lattices in the presence of the disorder leading to the Anderson localization. The disorder is created by interaction of the main fraction of atoms with the second immobilized fraction distributed randomly over the lattice. In low-dimensional systems there is no transition from the Anderson localized to the conducting phase, although in the presence of correlations a discrete set of extended states can exist. The first part of the thesis is devoted to properties of such states. In the finite size lattices, the presence of those states results in the appearance of `windows of transport' -- energy ranges, in which the localization length is longer than the system size.The analytical method of determining the extended states energies for correlations of generalized $N$-mers type is presented, along with a proof that indeed those states are extended in the infinite system. Subsequently, the way of experimental creation of this type of correlations is proposed, as well as the technique of generation of the disorder in the tunneling amplitudes, which significantly enhances tunability of the proposed energy filters. The second part of the thesis describes the method which allows to simulate a specific type of the disorder: random magnetic field. Systems of such a class, created in a two dimensional lattice using simultaneous fast periodic modulation of the lattice height and interactions with immobilized species, may allow in future to investigate the range of topics from the condensed matter physics, for example fractional quantum Hall effect at half-filling. In the thesis, the most interesting features observed upon investigation of such systems are presented. Especially, the anomalously low localization length for correlated disorder is explained.